1. Field of the Invention
The present invention relates to a cylinder block for a multicylinder engine and more particularly, to a cylinder block for a multicylinder engine having a cylinder main body in which multiple cylinder bores, each having a piston slide surface to slide a piston in an inner periphery, are provided in parallel in an axial direction to a crankshaft and a crankcase. The crankcase has multiple journal walls rotatably supporting at least a part of the crankshaft, integrally provided with the cylinder main body. Communication holes extending in parallel in the axial line of the crankshaft are provided in the cylinder main body and the crank case, while at least a part of the communication holes are opened in the inner peripheries of the cylinder bores.
2. Description of Background Art
Conventionally, such a cylinder block is already known, e.g., in Japanese Published Unexamined Patent Application No. Hei 11-182326.
Conventionally known is a structure where a communication hole connecting adjacent crankcases is provided in a cylinder block to prevent increase in pumping loss due to increase in pressure in the crankcase upon down movement of piston. A burr, which cannot be removed by honing process to form a piston slide surface in an inner surface of a cylinder bore, may occur at an open edge of the communication hole.
For this reason, in the above conventional art, the distance between the burr which occurs in the open edge of the communication hole and an oil ring at a lower end of the piston at the bottom dead center is set to 3 mm or longer. This configuration is set so as to prevent the increase in slide resistance by contact between the oil ring at the lower end of the piston and the burr at the open edge of the communication hole.
However, in the above-described dimensional setting, downsizing of the cylinder block is limited in a direction along the axial line of the cylinder bore, and freedom of positional setting of the communication hole is narrowed. Further, as the burr remains at the open edge of the communication hole, ventilation resistance of air flow through the communication hole increases. To sufficiently reduce the pumping loss, it is necessary to reduce the ventilation resistance of the air flow through the communication hole.
The present invention has been made in view of such situation. The present invention has its object providing of a cylinder block of a multicylinder engine which enables downsizing of cylinder block, increases freedom of positional setting of the communication hole, and reduces ventilation resistance of air flow through the communication hole.